C# SHA256 on files with very long file paths

Question

I get a List<string> of files via recursion through FileFinderEx via win32 api (DllImport of kernel32). (There's several questions that go over how this is done).

I handle fully qualified paths greater than MAX_PATH by using \\\\?\\UNC\\ and \\\\?\\ .

Also, from win32 api I can grab the "short path" (8.3 name) of file via GetShortPathNameW .

Now that I have the List<string> of fully qualified paths to files, I want to iterate over each path and place their SHA256 into another list (same index). So roughly, something like this:

List<string> files = new List<string>();
//win api function populates files ...
List<string> hash = new List<string>();
for (int i = 0; i < files.Count; i++)
{
    using (var stream = new BufferedStream(File.OpenRead(files[i]), 1200000))
    {
        SHA256Managed sha = new SHA256Managed();
        byte[] checksum = sha.ComputeHash(stream);
        hash.Add(BitConverter.ToString(checksum).Replace("-", String.Empty));
        stream.Close();
    }
}

The problem I face is even if I give it the "short path" (8.3 name) the File.OpenRead always throws exception if the fully qualified UNC name is greater than MAX_PATH (260 I believe).

So... now what? Is there some win32 api function I can import to do the reading instead of File.OpenRead ? Or is there some other .NET method to open files with long file paths? Any tips to get around this limitation?

Answer 1

Take a look at the CreateFile function in kernel32.dll. Despite it's name, you can use it to either read or create a file, and the Unicode version will allow you to use paths longer than MAX_PATH .

[DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
internal static extern SafeFileHandle CreateFile(
    string lpFileName,
    EFileAccess dwDesiredAccess,
    EFileShare dwShareMode,
    IntPtr lpSecurityAttributes,
    ECreationDisposition dwCreationDisposition,
    EFileAttributes dwFlagsAndAttributes,
    IntPtr hTemplateFile);

There is a good series of posts (part 1 here ) on the BCL Team's blog from a few years back covering the topic of long paths in Windows and .NET, which I think you'd find useful.